1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to the image forming apparatus in which a start up time of a fixing device is shortened.
2. Discussion of the Background
An electrophotographic or ink jet image forming apparatus generally fixes a developer onto a transfer sheet by heating the developer, such as toner or ink, by using a fixing device. In the electrophotographic image forming apparatus, a heater such as a heat roll is generally used as the fixing device. Various studies have been made to stably supply the fixing device with power.
Japanese Patent Laid-Open Publication No. 8-339134 discloses an image forming apparatus having a toner image forming device that forms a toner image on a transfer sheet, a fixing device that fixes the toner image onto the transfer sheet by an electromagnetic induction heating, a temperature controller that controls a temperature of the fixing device, and at least two protectors that stop energization of the fixing device when the fixing device reaches to a temperature equal to or out of a predetermined temperature range. Thus, the image forming apparatus includes two devices (i.e., control systems) that control the temperature of the fixing device to increase reliability of the fixing device.
Japanese Patent Laid-Open Publication No. 9-197856 discloses an induction heating fixing device that includes a heated member formed of a conductive member, a coil to inductively heat the heated member, a inverter circuit to supply the coil with a high frequency, a thermistor that detects a temperature of the heated member, an output control circuit (which is electrically insulated from the inverter circuit) to control the inverter circuit based on a temperature detected by the thermistor such that the temperature of the heated member is maintained within a predetermined range, and an insulating interface that transmits a control signal input from the output control circuit to the inverter circuit while electrically insulating the control signal. Thus, the induction heating fixing device is configured to control a temperature with a low temperature ripple by electrically insulating a first circuit from a second circuit.
In a conventional image forming apparatus, because a temperature is detected with single sensor, an abnormal temperature condition occurs. Moreover, in recent years, a demand for energy savings is increasing in an image forming apparatus. Thus, attempts have been made to save energy. For example, energization of a fixing device is cut off in a standby state, or the fixing device is maintained at a temperature lower than a fixing temperature in the standby state. The present inventors have recognized that in such an image forming apparatus having an energy saving function, a quick start up is required when an image forming operation is performed. However, no technology for shortening the start up time is discussed in the above-described Japanese Patent Laid-Open Publications.